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AROMATIC HYDROCARBONS

Aromatic hydrocarbons are those which contain one or more benzene rings. The name of the class come from the fact that many of them have strong, pungent aromas. Symbolic structures of some common aromatic hydrocarbons are shown below. Those hydrocarbons which do not contain a benzene ring are called aliphatic hydrocarbons.

A vast number of aromatic compounds can be produced by substitutions for one or more of the ring hydrogen’s.

Aromatic hydrocarbons have ring structures. Ring systems with double bonds that cannot be localized are called mesomere systems. Aromatic hydrocarbons carrying hydroxyl-groups are called phenols. There exist also ring compound with ring elements other than carbon. Such ring systems are termed heterocyclic rings. Heterocyclic rings are components of nucleotides that have an important role in the cell.

Benzene and its derivatives belong into this group, some of whose members occurring in plants are characterized by a strange, ‘aromatic’ smell. The benzene molecule has a ring-shaped structure with six C-H-groups that are linked alternately with C-C single and C=C double bonds. The structure should thus be written (according to KEKULE):

The electron theory distinguishes between two types of bonds in such a ring system, the sigma-bonds and the pi-bonds that are arranged vertically to each other and are conjugated so that the double bonds of such a system cannot be localized directly. Such a system is also called a mesomere system. There exist consequently six equal ‘aromatic’ bonds. A compound consisting of two benzene rings is naphthalene

Aromatic hydrocarbons with hydroxyl-groups are called phenols. They are able to dissociate in alkaline solutions and to form phenolates.